Cogging torque, which is non-uniform torque of a stator necessarily generated in a motor using a permanent magnet, refers to radial torque for moving to an equilibrium state, that is, a position at which magnetic energy of the motor is minimal.
An abrupt change in magnetic flux around a boundary between N and S poles of a magnet generates the cogging torque. The cogging torque causes noise and vibration and reduces the performance of a motor, and thus it is important to reduce the cogging torque. Particularly, it is very important to reduce the cogging torque of a motor used for an actuator for controlling a precise position.
Teeth of a stator and a magnet of a rotor are disposed to face each other, and cogging caused by a change in magnetic flux density may be generated in an edge area of the teeth of the stator. Particularly, when end surfaces of the teeth of the stators do not form a perfect circle, cogging torque may be generated.